1. Field of the Invention
This invention relates to a capacitance measuring device for measuring the capacitance of a capacitor to be measured with respect to the calibrated reference capacitance of a capacitor by means of the comparison of the charge and discharge periods of said capacitors during two successive measuring and reference phases of a comparison cycle. In particular, this invention concerns measurements of a physical quantity such as a displacement or a pressure generated from a capacitive sensor in which a variation of the physical quantity is converted into a capacitance variation.
2. Description of the Prior Art
Devices of the aforementioned type have been made to compare the capacitances of a capacitor to be measured and a reference capacitor. For example, the French patent application No. PV 73 45271 filed on Dec. 18, 1973 in the name of "Societe nouvelle de roulements" (S.N.R.) discloses a capacitance measuring device which is analogous to a zero-beat indicator. Such device comprises a counter for counting oscillation pulses delivered by an oscillating circuit which includes the reference capacitor or the capacitor to be measured during the reference phase or the measuring phase, and a counter-down-counter which counts and down-counts clock pulses at a predetermined frequency during each comparison cycle.
The count of the counter is equal to its predetermined maximum count n at the end of each of the successive reference and measuring phases. The zero-setting of the counter is automatically controlled at the end of each of the phases. In this way, the durations of the reference and measuring phases of a comparison cycle are different and are equal to .DELTA.t.sub.E =nT.sub.E and .DELTA.t.sub.X =nT.sub.X respectively, since the periods T.sub.E and T.sub.X of pulses delivered from the oscillating circuit are different therebetween and proportional to the capacitances of the reference capacitor and the capacitor to be measured respectively.
During the reference phase, the counter and the counter-down-counter count the oscillation pulses at the frequency F.sub.E =1/T.sub.E and the clock pulses respectively. When the count of the counter is equal to n at the end of the reference phase, it controls the change of the counting condition to the down-counting condition of the counter-down-counter. Consequently, the counter-down-counter counts and down-counts a same predetermined clock pulse number during a time T' equal to 2.DELTA.t.sub.E.
In this capacitance measuring device, the counter and the counter--down-counter deliver two pulses of respective durations T=.DELTA.t.sub.E +.DELTA.t.sub.X =n (T.sub.E +T.sub.X) and T'=2.DELTA.t.sub.E during each comparison cycle. The phase shift (T-T') is calculated in a phase comparison circuit so as to deduce the difference between the capacitances X.sub.E,X of the reference capacitor and the capacitor to be measured; the measuring device thus is analogous to a zero indicator.
As a result, measurement of the capacitance is affected by drifts in the oscillating circuit and interference capacitances especially between the capacitor electrodes and the ground referenced voltage, since the difference (T-T') is directly proportional to a factor K representing these drifts and interferences EQU T-T'=nK(X.sub.E +X)-2nKX.sub.E
or EQU T-T'=nK(X-X.sub.E)